The present invention relates to a phosphor paste and an insulative paste used for forming a luminescent layer or an insulator layer, respectively, of an electroluminescent (EL) element, and to an EL element having a luminescent layer or an insulator layer formed from said pastes by solidifying them with radiation.
An EL element comprises a luminescent layer formed by dispersing a pulverized EL phosphor in a binder made of an inorganic or organic material. The basic structure of EL element of AC driving system comprises an insulator layer (this layer is used to insulate electrically a transparent electrode and a back electrode) and a luminescent layer sandwiched between a back electrode and a transparent electrode across which a sine-wave AC or bipolar pulse voltage is applied so that the luminescent layer is placed under the action of a strong electric field to produce luminescence, which is emitted as light through the transparent electrode. Such EL element of AC driving system include the ones, though only rarely used, in which a luminescent layer alone is placed between the back and transparent electrodes.
When an organic material is used not only for the binder forming the luminescent or insulator layers of EL element but also for the transparent electrode and protective layer, it is possible to obtain an EL element which is thin, light in weight and high in impact strength and also has good flexibility.
The conventional method for producing said EL element of AC driving system comprises the step of applying an insulative paste mainly composed of a pulverized inorganic high dielectric substance such as barium titanate and a solidifiable compound to form an insulative paste layer, the step of applying a phosphor paste mainly composed of pulverized EL phosphor and a solidifiable compound to form a phosphor paste layer, and the step of solidifying said paste layers to thereby form an insulator layer and a luminescent layer. Regarding the way of effecting said solidification, there are known only two methods: solvent drying and heat setting.
In the case of the solvent drying type paste, a soluble polymer obtainable by polymerizing a monomer such as acrylic esters, epoxy resin, etc., or a cyanoethylated compound is used as binder. Said soluble polymer or cyanoethylated compound are solid or a viscous liquid, so that they need to be dissolved in a solvent for dispersing a pulverized phosphor therein.
In the case of the heat setting type paste, a heat curable compound resin is used as binder.
In the conventional method for producing EL element as described above, solvent drying or heat setting is required for forming an insulator layer or a luminescent layer, and such heating step, either drying or setting, takes about one to two hours. Also, in order to form the insulator layer or luminescent layers of a desired thickness, the application and drying operations or the application and setting operations may be required to repeat several times. Therefore, it is desired to shorten the solidification step. Further, the cyanoethylated compounds generally used as binder tend to be colored when exposed directly to the air under a high temperature condition for a very long time. This would reduce brightness of light emitted by the EL element and change the color of light emitted. Accordingly, it is desired to shorten even a little the time required for the solidification.
As an EL element using compound solidifiable by radiation, U.S. Pat. No. 4,513,023 discloses "a thin electroluminescent lamp assembly comprising forming a UV curable dielectric matrix by loading non-encapsulated particles of electroluminescent phosphor into a UV curable dielectric composition, depositing a coating of such composition over the surface of a transparent conductor, curing such composition by exposure to ultra-violet light in a substantially inert atmosphere," and "The phosphor particles may be loaded into any conventional UV curable dielectric composition such as, for example, the UV curable dielectric 5011D which is available from the Dupont Co. Inc. of Delaware U.S.A.".
The UV curable dielectric 5011D has a dielectric constant of less than 5 at 1 KHz and 25.degree. C.
The present inventors have devised a new way of solidification utilizing radiation instead of the conventional solvent drying or heat setting type solidification techniques in the production of EL element. Further, the present inventors found that in a preparation of EL element, it is not always necessary to use an insulative paste and a phosphor paste solidifiable by radiation in order to form both an insulator layer and luminescent layer, and the disadvantage of conventional preparation method can be markedly decreased when the insulative or the phosphor paste solidifiable by radiation is used for either of insulator layer or of luminescent layer.